Generally, nitrides of group III elements such as gallium nitride (GaN), aluminum nitride (AlN), and the like, have excellent thermal stability and a direct transition energy band structure and therefore, have greatly become of interest as materials for a light emitting device of visible and ultraviolet regions. In particular, blue and green light emitting devices using indium gallium nitride have been used for various applications, such as a large-scale full color flat panel display, a traffic light, indoor illumination, a high-density light source, a high resolution output system, optical communication, and the like.
A nitride semiconductor layer of the group III elements has been grown on a heterogeneous substrate having a similar crystalline structure by processes, such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and the like, since it is difficult to fabricate a homogeneous substrate on which the nitride semiconductor layer may be grown. A sapphire substrate having a hexagonal structure as a heterogeneous substrate has been mainly used. Recently, a technology of fabricating a high efficiency light emitting diode having a vertical structure by growing epitaxial layers such as a nitride semiconductor layer on a heterogeneous substrate such as sapphire, bonding a support substrate to the epitaxial layers, and separating the heterogeneous substrate using a laser lift off technology, and the like, has been developed. The heterogeneous substrate such as sapphire and the epitaxial layer grown thereon have different physical properties and therefore, the growth substrate can be easily separated using an interface therebetween.
However, the epitaxial layer grown on the heterogeneous substrate has a relatively high dislocation density due to a lattice mismatch and a difference in thermal expansion coefficients with the growth substrate. It has been known that the epitaxial layers grown on the sapphire substrate generally has the dislocation density of 1E8/cm2 or more. The epitaxial layer having the high dislocation density has a limitation in improving emission efficiency of the light emitting diode.
Further, since the entire thickness of the epitaxial layer is very thin at several micrometers as compared with an emission area of, for example, 350 μm×350 μm or 1 mm2, it is difficult to spread current. Further, as compared with the case in which the light emitting diode is operated at high current with the case in which the light emitting diode is operated at low current, current is concentrated at the dislocation, which leads to a droop phenomenon of reducing internal quantum efficiency.